Besides typical hydrophobic amino acids providing hydrophobic interactions, glutamine as a hydrophilic amino acid has also been known to be an important element in many self-assembling peptides, but it is still not clear how this particular amino acid contributes to the self-assembling process. We supposed that the dimethanediyl group in the side chain of glutamine could provide hydrophobic interaction for peptide self-assembly. To prove this hypothesis, we used the GNNQQNY peptide and its derivatives as examples to show the importance of the dimethanediyl group for peptide self-assembly. We found a very close relationship between the number of dimethanediyl groups, the strength of hydrophobic interaction, and the self-assembling ability of the peptides, indicating the hydrophobicity of the dimethanediyl group and its important role for self-assembly. This new finding might be instructive for clarifying the self-assembling mechanism of many natural peptides, as well as for developing novel self-assembling peptide nanomaterials.